Composite molecules model entities with attributes from different namespaces that are
not necessarily related. Each group of attributes is modelled by a molecule and the
"sub-molecules" are tied together with ~ methods to form a composite molecule.

The attributes of the first input sub-molecule awaiting 3 inputs are tied
together in a tuple of its own before being merged with the tuple of attribute values
of the second sub-molecule. If any of the sub-molecules are of arity-1, then no tuple is created: